1. Field of the Invention
The present invention relates to the termination of the conductor of a superconducting cable.
In the present description and attached claims, the expression “superconducting cable” is used to indicate a cable intended for carrying electric current in so-called conditions of superconductivity, that is, in conditions of almost zero electrical resistance under direct current transport condition.
In the present description and attached claims, the expression “conductor” is used to indicate the electrically active part of a superconducting cable, intended for carrying the phase electric current or that of each phase of a three-phase current system (where necessary, more in particular referred to as “phase conductor”). For the sake of brevity, and unless otherwise indicated, the expression “conductor” is also used to indicate the “return conductor”, that is, the electrically active part of a superconducting cable capable of transmitting the same quantity of electric current of the phase conductor/s associated with it, but in the reverse direction.
In the present description and attached claims, the expression “conductor termination” is used to indicate the connection to the conductor of an electrically conductive connector to allow fixing it to a second cable conductor, either superconducting or non-superconducting, or to an electrical apparatus in general, such as a transformer, an electrical motor, etcetera. In particular, in the case of fixing to a second conductor of superconducting cable, the termination in the above meaning must thus be intended as the formation of a joint between the two conductors. Moreover, for brevity, the electrically conductive connector shall sometimes be referred to as “top connector” in the following description.
A similar meaning is to be given to the terms “terminated conductor” and “terminator”.
2. Description of the Related Art
Warm dielectric (WD) superconducting cables and cold dielectric (CD) superconducting cables are known.
A warm dielectric superconducting cable (or each phase element of a warm dielectric three-phase cable) essentially comprises a tubular element for supporting one or more layers of superconducting tapes, and substantially defining a flow channel for a cryogenic fluid, a cryostat arranged coaxially external to the layers of superconducting cables, and a dielectric arranged coaxially external to the cryostat.
In the present description and attached claims, the expression “superconducting tapes” is used to encompass both types of superconducting material described hereinafter.
The expression “superconducting material” is used to indicate a material such as, for example, particular ceramic materials based on mixed oxides of copper such as those discussed by Cava R., J. Am. Ceram. Soc., 83 [1], 5–28 (2000). These compounds exhibit a substantially zero resistivity below a certain temperature, defined as critical temperature, or Tc. For example, the critical temperature for the above materials ranges between about 80K (−193° C.) and about 150K (−123° C.).
The superconducting material, in particular the BSCCO material, is commonly manufactured and used in the form of single- or multi-filament tapes wherein filaments of superconducting material are embedded in a metal matrix, usually silver, optionally added with aluminium or magnesium; or, in particular the YBCO and REBCO material is manufactured and used in the form of a film of superconducting material supported by a metal tape, and optionally coated with one or more oxide layers.
A cold dielectric superconducting cable (or each phase element of a cold dielectric three-phase cable) essentially comprises a tubular element for supporting one or more layers of superconducting tapes, and substantially defining a flow channel for a cryogenic fluid, and, arranged coaxially external to the layers of superconducting tape, in a sequence: a dielectric, a return conductor, an annular flow channel for the cryogenic fluid, and a cryostat. As an alternative, a single cryostat is provided for all phases present in the superconducting cable.
The tubular element for supporting the layers of superconducting tapes of the phase conductor can be at least partly made of a material exhibiting a low electrical resistance with the function of protecting the superconducting material from overcurrent, as described for example in the international patent application WO 00/39812 in the name of the Applicant. With the same function, in particular in the case of the return conductor, a screen external to the outermost layer of superconducting material can be provided, for example comprising one or more layers of conductive tapes, for example of copper.
In the present description and attached claims, the expression “cryostability device” is used to indicate such a tubular supporting element and/or such an external screen.
The operating temperature of a superconducting cable, a term used to indicate the temperature at which the superconducting cable transmits electric current in superconductivity conditions, is below the critical temperature of the superconducting material used.
For this purpose, as said, the superconducting cable is provided with at least one channel for the flow of a cryogenic fluid. The cryogenic fluid generally is helium, nitrogen, hydrogen and/or argon at application-specific temperature and pressure.
As known from the international patent application WO 01/08234 in the name of American Superconductor Corporation and of the Applicant, the prolonged contact of the superconducting tapes with the cryogenic fluid at the operating temperature and pressure, as well as the subjection to thermal cycles between such operating temperature and ambient temperature, can cause the infiltration of the cryogenic fluid into the superconducting tapes, with the consequent formation of “balloons”, which cause the deterioration of the superconducting tape performance.
To obviate the problem, that document describes a superconducting ceramic conductor for use in a cryogenic fluid, comprising a composite ceramic superconducting tape or wire and a sealing structure hermetically surrounding the outer surface of the composite ceramic tape/wire. In a first embodiment, the sealing structure is metallic and in particular, it comprises rolled metallic tapes on the greater faces of the superconducting tape, and non-porous solder fillets, for example of Pb—Sn—Ag, Pb—Sn, Sn—Ag, In—Pb, at the side faces of the superconducting tape. As an alternative, the solder can include dispersions of metallic fibres or particles in an epoxy resin.
In a different embodiment, the sealing structure comprises a polymer layer with optional metal elements dispersed therein, surrounding the outer surface of the superconducting tape or wire. The superconducting tape ends can be encapsulated through solder or silicone.